I just had a brainwave, although Im sure its probably not a good one.
So the original idea of trying to fit a disco body on a defender chassis wont work because its like trying to fit a lego brick on a mechano set, the bits dont match, and to make them match you need to weld stuff to the chassis and that IVA terratory. It also involves buing a defender and selling a big chunk of it, which is quite a waste of money if it can be avoided.
However, if you take a defender, or any old car, with a rusty chassis, you can replace that chassis with a brand new one, and its still the same car. (Note that if you repalce it with a chassis taken from another car, youre supposed to give the finnished car the identity of the car from which the chassis came from, and tell the DVLA about any engine/fuel/colour changes etc)
I realised that theres another way of putting a disco cab and a defender rear tub on the same LWB chassis - chop both the chassis in half and join them in the middle! Obviously, this is what is called a "cut-n-shut" and is completely illegal, but it would definately work because the main chassis rails are of the same specification, the same distance apart, etc etc.
So, here is what sprang to mind - what if I made (or more realistically, paid someone else to make for me) a brand new galv chassis, which is a 110 hicap or 130 at the back, and a disco at the front? Then Ive one new galv chassis, one chopped disco body, and a rear tub from a defender, an "of original type" engine/gearbox/etc, and they all fit together perfectly.
Now, some questions Ive got - First off, is this even legal? Or would it just be seen as a modified defender chassis and then need an IVA? Secondly, which car's identity would I use? Id want the higher payload of the defender, but of course, no part of that process actually requires buying a defender, just the rear tub, rear propshaft, and possibly the axle(s)? Along with a few other minor bits like wiring and brake lines.